This is especially relevant when attempting to reclaim near a frontline, with circles in order to get the necessary width you either need to go dangerously close to enemy territory or draw many smaller circles to approximate a rectangle.
The biggest design problem is how to handle the orientation of the rectangle (click and drag works fine for the dimensions), but I think just having the orientation fixed at 90 degrees (relative to the map) would generally be fine and still almost always better than a circle.